1. Field of the Invention
The present invention relates generally to a synchronous rectification switching regulator, a control circuit for a synchronous rectification switching regulator, and a method of controlling the synchronous rectification switching regulator.
2. Discussion of the Background Art
In recent years, a need has arisen to extend battery life in electronic devices that use a battery for power supply (e.g., PDA or personal digital assistance, cellular phone, etc). Battery life may be extended by enhancing a power supply circuit as well as by reducing wasteful power consumption.
In such electronic devices, a non-insulated switching regulator using an inductor is commonly used as an efficient power supply circuit. A synchronous rectification switching regulator that can enhance efficiency by using a MOS (metal-oxide semiconductor) transistor instead of a diode is widely used because the diode may cause a forward voltage drop.
However, in the case of the synchronous rectification switching regulator, electric current tends to flow in reverse (backflow current) in the inductor from an output terminal when a load current therein is low, thus decreasing efficiency. For example, a cellular phone has a low load current in a standby mode.
To prevent a backflow of the current, it is necessary to detect the backflow of the current. One type of synchronous rectification switching regulator includes a voltage comparison circuit as illustrated in FIG. 1. The current flowing in the inductor is converted to a voltage. The voltage comparison circuit detects the backflow of the current by comparing the voltage with a reference voltage.
In FIG. 1, a switching regulator 200 includes a backflow detection circuit 700, a PWM (Pulse Width Modulation) control circuit 701, a backflow prevention transistor 704, an inductor 705, a switching transistor 706 that is a PMOS (P-channel metal oxide semiconductor) transistor, a synchronous rectification transistor 707 that is an NMOS (N-channel metal oxide semiconductor) transistor, a resistance 708 having a resistance value Rsen, connected in series with the inductor 705, and a load 710. The resistance 708 converts an inductor current iL1 into a voltage. A voltage difference between both ends of the resistance 708 is Rsen×iL1. The backflow detection circuit 700 includes a voltage comparison circuit 702 and a latch circuit 703. The PWM control circuit 701 controls switching of the switching transistor 706 and the synchronous rectification transistor 707 to regulate an output voltage Vout.
FIGS. 2 and 3 are timing charts illustrating example wave patterns of respective parts shown in FIG. 1 when the load current is low. FIG. 2 illustrates a state in which the output voltage Vout is higher and the inductor current iL1 has a larger decrease slope. In FIG. 3, the output voltage Vout is lower and the inductor current iL1 has a smaller decrease slope.
In FIGS. 2 and 3, the voltage comparison circuit 702 detects an indication that the backflow current is about to occur when the voltage difference between both ends of the resistance 708 becomes an offset voltage Voff at P1. Because the voltage comparison circuit 702 has a delay time, the offset voltage Voff is set to a reference value that takes into consideration the delay time as well as changes in resistance value of the resistance 708, not to a value at which the backflow actually occurs. When the indication is detected, a signal indicating the occurrence of the backflow is transmitted to the latch circuit 703 after the delay time. Receiving the signal, the latch circuit 703 turns off the backflow prevention transistor 704 at P3 to shut off a path for the backflow of the current. The backflow prevention transistor 704 remains shut off until the PWM control circuit 701 turns on the switching transistor 706.
The inductor current iL1 flows in a forward direction (forward current) through a parasite diode of the backflow prevention transistor 704 for a time period DIO, after the backflow prevention transistor 704 is turned off until the forward current stops at P2. In FIG. 3, the time period DIO is longer than in FIG. 2. Therefore, the electricity loss caused by the parasite diode is larger.
Another type of synchronous rectification switching regulator, a step-down switching regulator, detects timing when a voltage at a connection between a switching transistor and a synchronous rectification transistor rises above a ground voltage after the voltage falls below the ground voltage at high speed. The switching regulator turns off the synchronous rectification transistor to prevent a current from flowing in reverse when a load is low.